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Geotechnieal Evaluation of Mine-Induced Seismleity on Monument Dam Dorsey 8 Whitney, LLP i 6.2.2 Water Pressures Water pressures for the "pre-failure" case were modeled based on available piezometer data for PZ-1 and PZ-3B that have been associated with threshold water levels for landslide movement (Norfleet, 1995). Water pressures in the landslide were estimated based on the following: • Water pressures in the landslide are mostly due to surface water infiltration and are largely independent of reservoir level (1Vorfleet, 1995}. • Steady-state seepage conditions were assumed for the landslide. • The upper landslide material can be represented as homogeneous and isotropic. • There are no internal drainage systems within the landslide mass and the influence of the constructed trench drain at the downstream toe was neglected. • The phreatic surface exits on the downstream slope above the toe of the landslide. 6.2.3 Material Properties • Weback-calculated the mobilized shear strength of the upper landslide material and the failure zone material based on the following: • The average effective mobilized cohesion of the failure zone is zero. ~J • Movement of the landslide was triggered by an increase in pore pressure caused by precipitation and seepage through the slope, which resulted in a phreatic surface at the threshold level. • Adjusting the drained friction angle of the upper landslide material and failure zone material until the computed factor of safety was equal to 1.0. The material properties of the less-disturbed lower landslide material were assigned based on engineering judgment to be somewhat stronger than the overlying upper landslide and failure zone material. The material properties used in the back-calculation analysis are summarized in Table 6.1. GEI 21 060350 0403-16 Movummt Dun GWtedu~ical Evil March 2006